1. Field of the Invention
The present invention relates to strain sensors which function with electro-conductive particles that are hereinafter called "conductive particle-polymer composites" and in the present invention, conductive particles are dispersed in a polymer such as plastics and rubbers in order to obtain conductive polymer composites, so that electro-conductive passes formed throughout the composites are used for measuring the increase of electric resistance in the composites that is brought by the elongation of the composites caused by an external force.
The sensors of the present invention can be used, for instance, for monitoring the safety of iron frames and iron-concrete of buildings. Nowadays, buildings, bridges, framed-up roads, tunnels, dams, etc. are mainly constructed with iron frames and iron-concrete; and for countries in which earthquakes often occur, it is desirable to monitor the iron frames and iron-concrete of buildings, bridges, tunnels, and so on. With the use of the sensors of the present invention together with a computer network system, a real-time on-line monitoring of earthquakes is attainable.
The sensors of the present invention can be used not only for the constructions described above but also for monitoring the unbalanced sinking of tanks of the chemical industry by way of on-line monitoring. Furthermore, an inspection of hazardous material storing facilities such as underground tanks which are the subjects of regulations regarding the handling of hazardous material can be executed on regular working days without removing such materials from the tanks.
In addition to the example described above, the sensors of the present invention can be installed in ships, mega-floats, aircraft, and tank-trailers. Especially, a great contribution can be expected when used in the mega-floats, since the mega-floats presently need to be carried to docks for inspections; and by installing the sensors of the present invention, the number of uses of docks for inspections of the mega-floats can be reduced, which saves expenses.
2. Prior Art
Until the present, it has not been possible to inspect the damages of iron frames and iron-concrete without destroying some part thereof. Taking as an example, for the iron frames of a building, it is necessary to partly destroy the ceiling or floor and to peel off the fireproof coating from the surface of iron frames, and the damage can be seen only after these troublesome processes. Therefore, a new technique that allows an inspection of damages on iron frames and iron-concrete by using strain sensors without destroying any part of the building has been desired.
Presently, several strain sensors are known; and these are coils of resistor wire, resistors patterned on plastic films, ceramic sensors which make use of deformation of the ceramics and sensors which make use of cutting carbon fibers.
Printed sensors with a metallic powder ink looks similar to the sensor of the present invention at first sight. However, there are great differences between the two as described below.
In the case of sensors in which a conductive ink is printed, a deformation of the sensors causes a change in the resistance; and in this case, the resistance and the strain are proportional to each other. Generally, the relation between the resistance and the strain is approximately linear. Thus, a severe calibration is necessary when the sensors are installed in the building in order to determine damages caused by an earthquake. For the patterned sensors, not only the severe calibration but also a long term stability of the resistance is inevitable, because it is necessary to judge from the small variation of resistance whether the buildings are damaged or not. Since no one knows when an earthquake may occur and it may be 10 years or 100 years after the sensors are set, the stability of resistance must be effective for such a long period of time. Generally, calibration of sensors is done once per several years. It is not guaranteed that the resistance of the patterned sensors is stable over such a long period of time.
It is described above that a severe calibration is necessary for the patterned sensors. This requires that the thickness of the pattern should be constant throughout the entire sensor. This again limits the size of the sensor. Namely, it is easy to have a constant thickness for a smaller size than for a larger size. For a larger size, a great amount of severe process-control is necessary so as to obtain homogeneous properties over the entire region of the pattern. This is one of the reasons that patterned sensors of large sizes are not available.
Since printed layers of sensors consist of metal particles aggregate from ink, resistances of sensors are limited within a certain range. Volume resistances are not varied as desired. This is one of the reasons that bigger sizes are not available.
Beside the problem described above, the major disadvantage of the prior art patterned sensors is that they are small in size. Taking an example of buildings, a great deal of iron frames are used in a building. Thus, there are numerous points to be monitored. When small size sensors are used, numerous sensors are necessary to monitor a building. This results in extraordinarily high cost. This is a much harder problem for urban high ways. Therefore, the patterned sensors cannot actually be used for buildings, bridges, ships, mega-floats, urban high ways, etc.
Another disadvantage of the prior art sensors is "linear response", namely resistances increase linearly with increasing strain. Therefore, severe calibration and stability during long periods are inevitable for monitoring systems. The critical response must always be clear to determine the damages of the iron frames of buildings.